Inverters for converting DC power into AC power for AC loads typically include one or more inverter legs, each leg in turn having a pair of controllable switches connected in series across the terminals of a DC power source. The inverter switches, which may be bipolar transistors, insulated gate bipolar transistors (IGBT's) or the like, are operated by an inverter control circuit to produce a PWM output waveform. For a three-phase inverter, the outputs of three inverter legs are typically displaced 120.degree. with respect to one another.
Prior inverter controls have been developed which utilize PWM patterns that are stored in a memory and which are retrieved in accordance with a voltage magnitude command signal and a frequency command signal. Examples of such types of inverter controls are disclosed in Ito, U.S. Pat. No. 4,672,285 and Ito et al., U.S. Pat. No. 4,654,773. In at least the case of the Ito '285 patent, all of the control signals for the inverter legs are derived from a single voltage pattern provided at the output of a read only memory, and hence independent phase voltage regulation is apparently not possible.
Other types of inverter controls have been designed wherein a separate memory is provided for each inverter phase and wherein the memories are separately accessed to allow individual phase voltage regulation. An example of such a control is disclosed in Parro II, U.S. Pat. No. 4,595,976, assigned to the assignee of the instant application and the disclosure of which is expressly incorporated by reference thereto. The Parro inverter control includes 3 memories, each of which stores a plurality of half-cycle PWM waveform patterns. The memories are individually addressed so that appropriate half-cycle PWM control waveforms are obtained for each phase. The half-cycle waveforms are used in conjunction with half-cycle signals to derive the switch control signals required to reproduce an entire cycle of an inverter phase output.
Other types of inverter controls are disclosed in Shekhawat et al., U.S. Pat. No. 4,635,177, Glennon, U.S. Pat. No. 4,527,226 (both of which are assigned to the assignee of instant application), Japanese Patent No. 60-74969 to Tomita and Japanese Patent No. 57-55775 to Maekawa.